Multiple-input, multiple-output (MIMO) techniques can significantly increase system capacity in a scattering environment of a wireless network. However, multiple antennas increases hardware complexity and cost because, in a typical system, each transmit/receive antenna requires a separate radio frequency (RF) chain including a modulator/demodulator, an AD/DA converter, an up/down converter, and a power amplifier. In addition, the processing complexity at the baseband also increases with the number of antennas.
Antenna selection (AS) can reduce the number of RF chains while still taking advantage of the capacity/diversity increase provided by multiple antennas. The idea is to select a submatrix from a complete channel matrix according to some predetermined criterion. To perform antenna selection, the complete channel matrix is estimated by sending training frames to measure the complete channel state information (CSI). The training frames can be sent in the same physical layer packet or by multiple packets.
Conventionally, on transmitting or receiving these AS training frames, the device conducting antenna selection switches different antenna subsets to the RF chains and estimates the corresponding subchannels matrices. The selection is based on the complete channel matrix composed of the estimated subchannels matrices.
However, conventional antenna selection schemes ignore the fact that each possible connection of a RF chain to an antenna introduces one unique RF response containing the effects of both amplitude gain and phase shift. As a result, in some circumstances, distortions are inevitable for antenna selection, because the selected antennas used for data transmission and reception may be connected to RF chains different from those used during training.
In the data transmission phase, the actual channel associated with the selected antennas may not be identical to that used in the training phase. This phenomenon is known as a RF imbalance problem.
It is desired to correct the RF imbalance problem in MIMO systems.